October 26, 1999
Photo No: H99-40
Hubble Identifies Source of UV Light in an Old Galaxy

Hubble Space Telescope's exquisite resolution has allowed astronomers to
resolve, for the first time, hot blue stars deep inside an elliptical
galaxy. The swarm of nearly 8,000 blue stars resembles a blizzard of
snowflakes near the core (lower right) of the neighboring galaxy M32,
located 2.5 million light-years away in the constellation Andromeda.

Hubble confirms that the ultraviolet light comes from a population of
extremely hot helium-burning stars at a late stage in their lives. Unlike
the Sun, which burns hydrogen into helium, these old stars exhausted
their central hydrogen long ago, and now burn helium into heavier
elements.

The observations, taken in October 1998, were made with the camera mode
of the Space Telescope Imaging Spectrograph (STIS) in ultraviolet light.
The STIS field of view is only a small portion of the entire galaxy,
which is 20 times wider on the sky. For reference, the full moon is
70 times wider than the STIS field-of-view. The bright center of the
galaxy was placed on the right side of the image, allowing fainter
stars to be seen on the left side of the image.

Thirty years ago, the first ultraviolet observations of elliptical
galaxies showed that they were surprisingly bright when viewed in
ultraviolet light. Before those pioneering UV observations, old groups
of stars were assumed to be relatively cool and thus extremely faint in
the ultraviolet. Over the years since the initial discovery of this
unexpected ultraviolet light, indirect evidence has accumulated that it
originates in a population of old, but hot, helium-burning stars. Now
Hubble provides the first direct visual evidence.

Nearby elliptical galaxies are thought to be relatively simple galaxies
comprised of old stars. Because they are among the brightest objects in
the Universe, this simplicity makes them useful for tracing the
evolution of stars and galaxies.